Forbid Choice annotations being used with autocomplete parameters

Since the Choice are synthetic, the library doesn't have a
pre-populated list of choices to know what to use as the resulting
value.

discord/app_commands/commands.py

@@ -246,6 +246,11 @@ def _populate_autocomplete(params: Dict[str, CommandParameter], autocomplete: Di
         if param.type not in (AppCommandOptionType.string, AppCommandOptionType.number, AppCommandOptionType.integer):
             raise TypeError('autocomplete is only supported for integer, string, or number option types')
 
+        if param.is_choice_annotation():
+            raise TypeError(
+                'Choice annotation unsupported for autocomplete parameters, consider using a regular annotation instead'
+            )
+
         param.autocomplete = _validate_auto_complete_callback(callback)
 
     if autocomplete:
@@ -589,6 +594,11 @@ class Command(Generic[GroupT, P, T]):
             if param.type not in (AppCommandOptionType.string, AppCommandOptionType.number, AppCommandOptionType.integer):
                 raise TypeError('autocomplete is only supported for integer, string, or number option types')
 
+            if param.is_choice_annotation():
+                raise TypeError(
+                    'Choice annotation unsupported for autocomplete parameters, consider using a regular annotation instead'
+                )
+
             param.autocomplete = _validate_auto_complete_callback(coro)
             return coro
 

discord/app_commands/transformers.py

@@ -124,6 +124,9 @@ class CommandParameter:
 
         return base
 
+    def is_choice_annotation(self) -> bool:
+        return getattr(self._annotation, '__discord_app_commands_is_choice__', False)
+
     async def transform(self, interaction: Interaction, value: Any) -> Any:
         if hasattr(self._annotation, '__discord_app_commands_transformer__'):
             # This one needs special handling for type safety reasons